The present invention relates to an ignition timing control apparatus for an internal combustion engine which is particularly suitable for controlling the ignition timing of a marine engine.
FIG. 7 shows a circuit diagram of a conventional ignition timing control apparatus for a marine engine. The conventional ignition timing control apparatus illustrated includes an ignition device I and an ignition controller II for controlling the ignition device I. The ignition device I includes a generator coil 101 of a magneto generator which generates an alternating current output in sync with the rotation of an engine, a rectifier diode 102 for rectifying the alternating current output of the generator coil 101, and a diode 103 having an anode connected to ground and a cathode connected to a junction between the generator coil 101 and the rectifier diode 102. Ignition device I also includes a capacitor 104 which is charged by the output of the generator coil 101 through the rectifier diode 102, an ignition coil 105 having a primary winding 105a connected to the capacitor 104 and a secondary winding 105b, a spark plug 106 connected to the secondary winding 105b of the ignition coil 105 so as to generate a spark when a high voltage develops across the secondary winding 105b, and a semiconductor switching element 107 in the form of a thyristor for controlling the ignition coil 105. The thyristor 107 has an anode connected to a junction between the rectifier diode 102 and the capacitor 104, a cathode connected to ground and a control gate connected to the ignition controller II and to ground through a resistor for controlling the conduction of the thyristor 107.
The ignition controller II includes a crank position sensor 108 in the form of a signal generating coil installed, together with the generator coil 101, on the unillustrated magneto generator for generating an alternating positive and negative output in sync with the rotation of the engine, the alternating output being input to a set terminal S of a flip-flop 111 through a rectifier diode 110. As clearly shown in FIG. 8, the output signal of the signal generator 108 thus rectified by the rectifier diode 110 contains a plurality of triangular pulses of which peaks represent a predetermined crank angle or piston position of a cylinder corresponding, for example, to a maximum advanced ignition angle T.sub.1. The flip-flop 111 has an output terminal connected through a resistor 112 and a capacitor 113 to an inverted or negative input terminal of a comparator 116 which has a non-inverted or positive input terminal imposed with a reference voltage V.sub.2. An operational amplifier 114 has an inverted or negative input terminal connected to a junction between the resistor 112 and the capacitor 113 and a non-inverted or positive input terminal imposed with a reference voltage V.sub.1, and an output terminal connected to a junction D between the capacitor 113 and the negative input terminal of the comparator 116 and at the same time to an inverted or negative input terminal of a comparator 115 which has a non-inverted or positive input terminal connected to ground and an output terminal connected to a reset terminal R of the flip-flop 111.
A pulse detecting circuit III includes a capacitor 117 having one end connected to an output terminal of the comparator 116 and the other end to the control gate of the thyristor 107 of the ignition circuit I, and a diode 118 having a cathode connected to the other end of the capacitor 117 and an anode connected to ground. A junction B between the rectifier diode 110 and the set input terminal S of the flip-flop 111 is connected to a junction G between the control gate of the thyristor 107 and the capacitor 117 through a resistor 121 and a diode 119. A junction F between the resistor 121 and the diode 119 is connected to a junction E between the output terminal of the comparator 116 and the capacitor 117 through a diode 120.
FIG. 8 illustrates the operational wave forms at various portions of the ignition controller II wherein reference character A designates the crank angle or position of the engine in which TDC designates top dead center, T.sub.1 a maximum advanced ignition angle or position at which a pulse b is generated by the signal generator 108, and M a required or target ignition angle or position at which ignition should take place; and reference characters Bv through Gv designate the wave forms of voltages or pulses at points B through G, respectively, in FIG. 7.
FIG. 9 is a diagram showing the ignition characteristic of the conventional ignition timing control apparatus of FIG. 7. As shown in FIG. 9, the ignition timing or position in the conventional apparatus is predetermined such that it is advanced at a predetermined constant rate or slope as the rotational speed or the number of revolutions per minute of the engine increases to a predetermined level N.sub.1. It is constantly held of a maximum advance angle T.sub.1 when and after the number of revolutions per minute of the engine has reached the level N.sub.1.
The operation of the conventional ignition timing control apparatus as constructed above is as follows. First, let us consider the case in which the engine is operating at a low rotational speed N which is less than or equal to the predetermined rotational speed or the predetermined number of revolutions per minute N.sub.1 (i.e., N.ltoreq.N.sub.1) of FIG. 9. In this case, the signal generator 108 generates, at the maximum advance angle position T.sub.1 for every revolution of the engine, an angular or positional pulse b in the form of a triangular pulse which has a narrow angular width and sharply changes. When the angular pulse b is fed through the diode 110 to the set terminal S of the flip-flop 111, the output of the flip-flop circuit 111 becomes high so that the capacitor 113, which has already been charged with a predetermined polarity as shown in FIG. 7, begins to discharge at a rate of current I.sub.2 which is given by the following formula; ##EQU1##
As the capacitor 113 discharges at the rate of current I.sub.2, the output voltage Dv of the operational amplifier 114 decreases linearly at a constant slope as shown in FIG. 8, and when it reaches the ground potential at the positive or non-inverted terminal of the comparator 115, a positive pulse voltage is generated at the output terminal of the comparator 115. The positive pulse voltage is input to the reset terminal R of the flip-flop 111 so that the voltage at the output terminal thereof becomes low. As a result, the capacitor 113 begins to charge at a rate of current I.sub.1 which is given by the following formula; EQU I.sub.1 =reference voltage V.sub.1 /the resistance of resistor 112.
As can be seen from the above formulae, the charging and discharging currents I.sub.1, I.sub.2 of the capacitor 113 is constant even if the number of revolutions per minute of the engine changes under the condition that the high-level voltage at the output terminal of the flip-flop 111, the resistance of the resistor 112 and the reference voltage V.sub.1 are all held constant. Accordingly, the charging and discharging voltages of the capacitor 113 and hence the output voltages Dv of the operational amplifier 114 fall and rise linearly at predetermined constant rates or slopes, respectively, irrespective of the number of revolutions per minute of the engine, as clearly shown by Dv in FIG. 8. In this manner, the output voltages Dv of the operational amplifier 114 begins to fall at a constant slope, which is determined by the discharging current I.sub.2, from the maximum advance angle position T.sub.1 at which an angular pulse b is generated by the signal generator 108. After the output voltage Dv has decreased to the ground voltage at the positive or non-inverted terminal of the comparator 115, it again begins to rise at a constant slope which is determined by the charging voltage I.sub.2. The output voltage Dv of the operational amplifier 114, which changes in this manner to provide a triangular output voltage, is input to the negative or inverted input terminal of the comparator 116 where it is compared with the reference voltage V.sub.2 at the positive or non-inverted input terminal thereof. As a result of this comparison, the comparator 116 generates a high-level output voltage Ev during the time when the output voltage Dv of the operational amplifier 114 is lower than the reference voltage V.sub.2. The output voltage Ev is differentiated by the pulse detecting circuit III to provide a trigger voltage Gv as shown in FIG. 8. In order words, the capacitor 117 is charged by the rising or increasing output voltage Ev of the comparator 116 with a polarity as shown in FIG. 7, the charging voltage Ev generating, at location M in FIG. 8, a trigger voltage Gv for triggering the thyristor 107, as shown in FIG. 8. The charged energy of the capacitor 117 is discharged through the diode 118 during the low-level output of the comparator 116 and becomes ready for the next operation. Although the angular signal b of the signal coil 108 is input through the diode 119 to the control gate of the thyristor 107, when the number of revolutions per minute N of the engine is less than or equal to the predetermined value N.sub.1, the instant at which the signal coil 108 generates an angular pulse b corresponds to the time when the output of the comparator 116 is at the low level, so the angular pulse b is absorbed into the low-level output of the comparator 116 through the diode 120 and thus it is not supplied to the control gate of the thyristor 107.
On this occasion, the trigger voltage Gv of the comparator 116 is fed to the control gate of the thyristor 107 which is thus made conductive at a crank angle or position M, thus permitting the capacitor 104 to discharge through the primary winding 105a of the ignition coil 105 whereby a high voltage is induced across the secondary winding 105b of the ignition coil 105, causing the spark plug 106 to generate a spark. As evident from the foregoing description, it will be understood that in the rotational range of the engine in which the number of revolutions per minute N thereof is less than the predetermined value N.sub.1 in FIG. 9, the point in time at which the discharging output voltage Dv of the capacitor 113 reaches the reference voltage V.sub.2 of the comparator 116 becomes the ignition instant at which the cylinder is fired.
Next, the case in which the number of revolutions per minute of the engine N increases above the predetermined value N.sub.2 in FIG. 9 (i.e., N&gt;N.sub.1) will be described.
As the number of revolutions per minute of the engine increases toward N.sub.1, the advance angle .alpha. proportionally decreases toward zero. That is, the instant at which a trigger voltage Gv develops advances toward the maximum advance angle position T.sub.1 in proportion to the increasing rotational speed of the engine, and as the rotational speed of the engine further increases above N.sub.1, the advance angle .alpha. becomes zero. Though the number of revolutions per minute of the engine at this time actually becomes that at the final advance angle, as the rotational speed of the engine further increases above N.sub.1, the output voltage Dv of the operational amplifier 114 always remains below the reference voltage V.sub.2 as clearly shown in FIG. 8 (in the range of N&gt;N.sub.1), so the output voltage Ev of the comparator 116 is accordingly held at the high level. As a result, there will be no trigger voltage Gv developed in the pulse detecting circuit III.
On the other hand, an angular pulse b generated by the signal coil 108 at the maximum advance angle position T.sub.1 is input to the output terminal of the comparator 116 through the resistor 121 and the diode 120. If the number of revolutions per minute of the engine N exceeds the predetermined value N.sub.1, the angular pulse b is imposed, as a trigger voltage Gv shown in FIG. 8, on the control gate of the thyristor 107 through the diode 119 to trigger the thyristor 107 into the conductive state since the output voltage Ev of the comparator 116 is always at the high level. In other words, at the high rotational speed of the engine above N.sub.1, the angular pulse b generated at the maximum advance angle position T.sub.1 becomes a trigger signal for determining the ignition timing of the engine, so that the ignition constantly takes place at the maximum advance angle position T.sub.1 irrespective of the increasing rotational speed of the engine.
With the above-described conventional ignition timing control apparatus, the target ignition timing M is determined by the discharging time of the capacitor 113 measured from the maximum advance angle position T.sub.1 corresponding to the occurrence of an angular pulse b. In this case, if the rotational speed of the engine is constant, the discharging time corresponds exactly to a prescribed crank angle .alpha.. However, the engine repeatedly performs combustion, exhaust, intake and compression in a cyclic manner, so even within one revolution of the engine, the rotational speed thereof generally varies greatly. Accordingly, even if the discharging time of the capacitor 113 is held constant, the corresponding crank angle .alpha. changes depending upon variations in the rotational speed of the engine. In particular, the above variation in the rotational speed of the engine becomes remarkable in the low rotational speed range, thus preventing highly accurate ignition timing control.